The wetting behavior of a liquid on a solid surface is a phenomenon of significant practical importance. The angle of liquid-to-solid contact is important in diverse areas of science and technology such as adhesion, adsorption, lubrication, catalysis, solid-liquid reaction kinetics, heat transfer, electrical conduction, and micro-fluidic devices. The angle of contact is one way to measure and assess the phenomenon of liquid wetting of a solid surface.
The contact angle of a liquid on a surface may be used to define to what extent, if any, a liquid will “wet” or contact a surface. Whenever a liquid contacts a solid surface, several different types of behavior can be exhibited. At one extreme, a drop of liquid contacting a solid surface will spread out until it forms a thin film on the surface. This is called total wetting and in this case the liquid has a contact angle of zero with the surface. At the other extreme, a drop of liquid will sit on the surface like a marble with minimal contact. This behavior is termed non-wetting and the liquid in this case forms a contact angle of 180° with the surface. For situations in between these extremes, a drop will be formed that makes a well-defined contact angle, θ, with the surface. This is called partial wetting.
The static contact angle between a liquid and a smooth planar horizontal surface is commonly referred to as being the intrinsic contact angle. This contact angle is only dependent on the material properties of the liquid and the smooth horizontal planar surface. FIGS. 1a, 1b, and 1c in the drawings illustrate drops that exhibit different contact angles. FIG. 1a shows an intrinsic contact angle, θ, between 0 degrees and 90 degrees, i.e., of about 45 degrees. FIG. 1b shows an intrinsic contact angle of about 90 degrees. FIG. 1c shows an intrinsic contact angle greater than 90 degrees but less than 180 degrees.
The apparent contact angle is the static contact angle between a liquid and a horizontal surface with contamination, imperfections, and/or roughness (with the roughness being on a scale that is small compared to the size of the drop). In contrast to both the intrinsic and apparent contact angle, the dynamic contact angle is measured on a drop that is changing size or position, and is not necessarily on a horizontal surface.
For all these different types of contact angles, the standard historical convention applied to the partial wetting behavior in FIG. 1 states that if the contact angle is less than 90 degrees, the liquid “wets” the surface. If the contact angle is greater than 90 degrees, the liquid “does not wet” the surface and is termed “non-wetting”. In this application, the terms “wetting” and “non-wetting” will be used to refer to this partial wetting behavior and not to the absolute definitions.
Because the wettability of liquids on solid surface is important to quantify, there have been many approaches used to measure the contact angle of a liquid on a solid surface. Prior art measurement approaches have included the sessile drop method, the tilting plate method, the Wilhelmy plate method and the capillary rise method. Typically, the wettability of a surface is determined largely by the intrinsic contact angle, θ, that the liquid makes with the solid surface. The tilting plate method may be difficult to perform if only small amounts of liquid are available. The other methods typically require expensive goniometer-mounted tele-microscopes to accurately view the contact angle optically. These techniques may also have difficulty measuring the contact angle, θ, to an accuracy of better than 5 degrees. These techniques may also have difficulty measuring dynamic changes in the contact angle, θ. In addition, some of these techniques require expensive computer software to analyze the liquid interface and obtain a desirable accuracy of one percent in the measurement of contact angle.
In the prior art, the contact angle that the liquid makes with the solid surface is determined directly in order to determine the wettability of a surface. In many cases this is very difficult to perform due to the size of the drop. In the present invention, the contact angle of a liquid with a surface or a portion of a surface is determined indirectly by observing whether the liquid is able to fill or not fill an angular feature having at least two opposing sides with a known included angle between them. This angular feature is fabricated from the material in question. This is much easier to accomplish than observing the contact angle directly.
With the expanded need in the scientific and technical community to measure the contact angle of liquids on various surfaces, there is a need for a rapid and inexpensive means to measure both the static and dynamic contact angles of liquids on solid surfaces to one percent accuracy that will be accessible to any size laboratory, institution, or business.